The present invention relates generally to integrated circuits and in particular the present invention relates to cascode operational amplifiers.
Operational Amplifiers (OP AMP) are key elements to most analog circuits. Various amplifier configurations, such as common emitter (xe2x80x9cCExe2x80x9d), common base (xe2x80x9cCBxe2x80x9d) and common collector (xe2x80x9cCCxe2x80x9d) are available. The CE-CB configuration, usually referred to as a cascode amplifier, has basically the same input characteristics as a CE amplifier and the same output characteristics as the CB amplifier. Specifically, it has high output resistance, which is useful in achieving a large amount of voltage gain. Moreover, in this configuration, no high frequency feedback occurs from the output to the input, and an input Miller capacitance effect is at a minimum because the voltage gain of the CE configuration is very low. Therefore, the cascode configuration is inherently stable and hence ideally suited for high-frequency applications.
Referring to FIG. 1, a circuit diagram of a conventional fully differential folded cascode CMOS OP AMP is illustrated. The OP AMP 1 has two input voltage terminals (Vin+, Vinxe2x88x92), two output voltage terminals (Vo+, Voxe2x88x92), two power supply terminals (Vdd, Vss), four DC bias voltage terminals (A, B, C, D), a common mode feed back voltage terminal (VCM), seven NMOS transistors (M1, M2, M3, M8, M9, M10, M11), and four PMOS transistors (M4, M5, M6, M7).
Increasing the bandwidth and open loop gain of an OP AMP for use in high speed switching circuits can be difficult. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for fully differential folded cascode operational transconductance amplifier with enhanced gain and bandwidth.
The above-mentioned problems with amplifiers and other problems are addressed by the present invention and will be understood by reading and studying the following specification.
In one embodiment, a differential amplifier comprises differential output nodes, first and second current sources coupled to the differential output nodes, and first and second current sinks coupled to the differential output nodes. A first pair of differential input transistors are coupled to the first and second current sources to adjust output voltages on the output nodes in response to input signals. A second pair of differential input transistors are coupled to the first and second current sources to adjust currents provided by the first and second current sources in response to the input signals.
The differential amplifier can include common mode circuitry coupled to the first and second current sources to further adjust the currents provided by the first and second current sources in response to a common mode voltage of the input signals.
In another embodiment, a differential cascode amplifier comprises input connections to receive differential input signals, output connections to provide differential input signals, and first and second current sources coupled to the output connections. A first pair of differential input transistors are coupled to sink current from the first and second current sources in response to the input signals. A second pair of differential input transistors are coupled to adjust currents provided by the first and second current sources in response to the input signals.
In yet another embodiment, a differential cascode amplifier comprises first and second input connections to receive differential input signals, first and second output connections to provide differential input signals and output circuitry coupled to the first and second output connections. The output circuitry comprises a first current source transistor electrically coupled to the first output connection, a first current sink transistor electrically coupled to the first output connection, a second current source transistor electrically coupled to the second output connection, and a second current sink transistor electrically coupled to the second output connection. The amplifier further includes differential circuitry coupled to the output circuitry. The differential circuitry comprises a first input transistor electrically coupled to the first output connection, a second input transistor electrically coupled to the second output connection, and a tail current transistor coupled to the first and second input transistors. The first and second input transistors sink current from the first and second output connection through the tail current transistor in response to the input signals. A dynamic bias circuit coupled to gates of the first and second current source transistors.
A method of operating a differential amplifier circuit comprises providing source currents to first and second differential output connections in response to first and second differential input signals, and adjusting current levels of the source currents in response to the first and second differential input signals.